About getter and setter - http://blog.milesbarr.com/2006/09/getters-and-setters/

Discussion of hashcode and equal of collections - http://redsolo.blogspot.com/2006/11/why-no-equals-and-gethashcode-in.html

A blog discuss the problem of getter / setter - http://jroller.com/page/vprise?entry=the_case_against_the_property

Equality Is Hard - https://www.craigstuntz.com/posts/2020-03-09-equality-is-hard.html